Many composite materials are commercially available which consist of a second component, for example a filler, distributed within a first component, referred to as the matrix material. In general terms, the filler component comprises one or more materials having mechanical, thermal, electrical or magnetic properties that differ significantly from those of the matrix material. When the components are combined, a composite material is formed that has physical properties unlike those of either the filler or the matrix. The properties of the composite material can be varied and controlled by changing the relative amounts or volume fractions of the components. If the filler component is randomly distributed within the matrix then, typically, the physical properties of the composite are independent of the orientation of one component within the other of the material. Such a material is said to have isotropic properties. Alternatively, if the spatial distribution of the filler component is controlled, then it is possible to make composites having physical properties that vary according to the orientation of the material. Structural fibreglass composites are a good example of the latter. The glass fibres can be deliberately aligned, parallel to the direction of an applied force, in order to take advantage of the relatively high tensile strength and stiffness that they have in comparison to the matrix. This composite is stronger in one direction and is said to have anisotropic properties.
In ‘Connectivity and Piezoelectric-Pyroelectric Composites’ Mat. Res. Bull. 13 pp525–536 (1978), Newnham R. E et al, the descriptive term connectivity was used to classify the distribution of the constituents in composite materials. An index number is assigned to each component according to the number of dimensions in which it is physically self-connected. A thin rod is said to be self-connected in one dimension and no other. A continuous matrix is self-connected in three dimensions. Using this terminology, a composite consisting of an array of aligned rods of filler material held by a continuous matrix is referred to as a 1-3 composite. Where individual filler particles are uniformly dispersed in a continuous matrix, they are said to be physically self-connected in no dimension. This type of material is referred to as a 0-3 composite.
Piezoelectric composite materials, consisting of a ferroelectric ceramic in an electrically-inactive polymer matrix, provide an example whereby the spatial distribution of the second component has a fundamental influence on physical properties. Both 0-3 type and 1-3 type composites are routinely used for passive sensors. The 0-3 type have adequate sensitivity and have a cost advantage due to their simplicity of manufacture. Their greater flexibility and formability make them the preferred choice for large area applications such as sonar. Composites with an 1-3 type connectivity are far more sensitive, but are also more expensive to make. They are preferred for active devices and for array-type sensors. Piezoelectric arrays are used for such applications as acoustic imaging and for medical ultrasound.
In array-type composites or 1-3 type composites, two of the key factors affecting properties are the size of the individual elements of the second component and the distance between those elements, referred to as the periodicity. In production, steps must be taken to ensure that these factors are controllable within specified limits. A commonly used technique utilises precision micro-machining to accurately cut individual array elements from a solid ceramic block. Arrays have also been made by injecting ceramic powder, or powder in suspension, into a mould under pressure. The resulting pre-form of the composite structure must then be sintered to consolidate the ceramic. In an alternative method, rods or ‘fibres’ can be aligned mechanically, by hand or otherwise, to produce a desired aligned structure. In practice this is difficult to achieve with the required precision due to the small scale of operations. Typically, the useful diameter of the rods lies in the range 30 to 400 microns. Once the desired structure of elements of the second component has been created it can then be bonded or embedded in a polymeric matrix to complete the composite material. Most commonly, a liquid pre-polymer is allowed to permeate the aligned structure and subsequently solidify. The term pre-polymer refers here to a compound or mixture of compounds that can undergo a chemical reaction to produce a polymeric solid. Alternatively, it is conceivable that a molten polymer could be allowed to permeate the aligned structure and then to solidify. Surface-active ‘coupling agents’ can be used to improve the useful properties of the composite by chemically modifying the interface between the filler and the matrix components.
In contrast, the production of 0-3 type composites is more straightforward. The filler or second component, in powder form, is mixed intimately with the first component, a liquid polymer or pre-polymer. The liquid wets the filler particles, before being made to polymerize or otherwise solidify. High volume fractions of filler are used and the composite is shaped by hot-pressing or warm-rolling. Ostensibly, a commercial 0-3 type composite consists of individual filler particles, each one being completely surrounded by a layer of the matrix material. In practice, the action of rolling or pressing often brings the particles into such close proximity that direct electrical contact may occur.
One method that could be used to induce a predetermined spatial distribution of the filler component in composites is referred to as dielectrophoretic assembly or electric-field structuring. In this process, a dispersion of filler particles in a liquid polymer or pre-polymer is exposed to a moderate a.c. electric field. Under suitable conditions, the filler particles become polarized and exhibit a mutually attractive force, which causes them to form chain-like structures between the electrodes. The liquid is then solidified by means of a chemical reaction or a change in temperature and the newly-formed structures are thereby fixed in place to form a composite material with anisotropic properties. This method has the potential advantage that materials having the sensitivity of 1-3 type composites could be made, whilst retaining some of the simplicity of the manufacture of 0-3 type composites.
The electric-field structuring technique utilises this dielectrophoretic force, which is responsible for an electrorheological effect. This is discussed in ‘Induced Fibrillation of Suspensions’. Journal of Applied Physics 20 pp 1137–1140 (December 1949), Winslow W. M. and ‘Dielectrophoresis: The Behaviour of Neutral Matter in Non-Uniform Electric Fields’. Cxnbridge University Press (1978), Pohl H. A. Various parameters affecting this are: the dielectrophoretic or polarization force, which is directed to produce the desired particle structure; viscous drag in the fluid, which resists particle motion; and sedimentation, which must be controlled. Alternating electric fields are used, by preference, to avoid electrophoresis. Applied electric-field strength is deliberately moderated to suppress such effects as electrically-induced turbulence in the fluid and accelerated curing of the polymer. Applied electric-field frequency is dictated by the dielectric properties of the fluid and the filler.
One of the major pitfalls associated with the electric-field structuring technique is sedimentation. Where particles of the filler component have a higher density than the surrounding fluid component, then they will fall out of suspension under the influence of gravity. The rate of sedimentation depends on particle size and shape and also on the viscosity of the surrounding fluid. In practice, the magnitude of the dielectrophoretic force can be set to overshadow viscous drag and also the effect of gravity. However, the forces acting on different sized particles are not of the same magnitude. This makes precise control over the shape of the electric-field-induced structures difficult and irregularities commonly occur. A further difficulty concerns the viscosity of the surrounding fluid, which is not constant over the course of the processing cycle. For example, thermosetting polymers such as epoxy resins exhibit a progressive increase in viscosity with time as polymerization proceeds. At the same time, the polymerization reaction itself is exothermic and generates heat. The fluid experiences a rise in temperature and consequently its viscosity decreases. Furthermore, the rate of reaction is increased at the higher temperature. These competing effects make precise control over fluid viscosity difficult to achieve. Accordingly, the rate of sedimentation of suspended particles is often uncertain.
In common with many composites, the interface between the filler and matrix components has a controlling influence on the physical properties of materials produced by electric-field structuring. The surface electrical properties of the filler particles, in particular, are of prime importance. Normally particles are completely surrounded by a layer of adsorbed polymer, giving true random 0-3 type connectivity within the matrix. Where these layers form insulating barriers between the particles, the useful electrical properties of the composite material can be adversely affected. Furthermore, where variations in the sizes of individual particles exist, chain branching in the field induced structure is found to occur. Anisotropy in the electrical properties will then be a function not only of the amount of filler, but also of the degree of this chain branching. In practice, some disparity in the spatial distribution of the filler particles is found to occur between otherwise identical composite samples. Hence, significant variability in the physical properties of composites prepared by electric-field structuring can normally be expected.